The present invention relates to a process for purifying metals by segregation, which can be applied in particular to aluminum.
In different technical fields, and in particular in regard to the production of high and medium voltage type electrolytic capacitors, there is an increasing tendency to use aluminum which contains only a few parts per million of impurities such as iron and silicon.
It is known that aluminum which is produced by the electrolysis of alumina may have a level of impurities concentration of the order of several hundreds of parts per million. It has, therefore, been found necessary to be able to provide processes which permit very pure aluminum to be produced.
One of those processes comprises refining the aluminum in the presence of copper by a process referred to as "three-layer electrolysis." However, that process suffers from the disadvantage of being relatively expensive to work and not always effective to produce a sufficient degree of purity in respect of certain elements, in particular iron and silicon.
Another process involves segregation, which operation comprises effecting cooling to cause partial crystallization of a molten metallic mass, then separating the solid phase, which is purified in respect of certain elements referred to as eutectic elements, from the liquid phase which is enriched in respect of the same elements.
The present invention lies in the second line of development. Eutectic elements such as iron, silicon, copper, magnesium and zinc all have the characteristics of forming with aluminum and with a given level of concentration, alloys which are referred to as eutectic alloys. The solidification temperature of these alloys, when they are cooled from the liquid state, is lower than that of pure aluminum.
If liquid aluminum containing such elements in amounts less than the eutectic concentration is subjected to cooling, it is known that the aluminum crystals which appear within the mother liquor are purer than the latter. Such a phenomenon is described by means of liquid-solid binary equilibrium diagrams which indicate, in the temperature range in which the two phases exist and for a given base metal-eutectic element pair, the concentration C.sub.S at which that element occurs in the solid when it is in condition of equilibrium with a liquid in which it occurs in a level of concentration C.sub.L. The ratio k=C.sub.S /C.sub.L which is lower than 1, referred to as the partition coefficient, is characteristic of each eutectic element and has very little dependency on its concentration in the alloy.
In practice, if C.sub.O is used to denote the concentration in the metal to be purified in respect of each of said impurities, a product in which that concentration is adjusted to C is obtained in the purified phase. The level of efficiency of the operation is generally indicated by the purification coefficient corresponding for each impurity to the ratio C.sub.O /C.
Processes based on such principles have already been patented. In the embodiments of those patents, it is found that the purification coefficients achieved are higher than those which are deduced from the partition coefficients. This results, surprisingly, because, when those processes are applied, they use complementary means, the effect of which is to modify the equilibrium states and thus improve the purification effect.
In particular, U.S. Pat. No. 3,303,019 filed in 1964, concerns a process wherein a molten mass of aluminum is poured into an unheated container, the side walls and the bottom of which are so designed as to limit thermal losses. The container is open in its upper portion, the bottom thereof is substantially flat, the side walls are vertical, and it is provided with a tapping orifice. The dimensions of the container are such that, for a charge of 700 kg, the metal occupies a height of 37.5 cm and has an area of contact with air of 8700 cm.sup.2, giving a ratio of 4/1000 between those two parameters. By removing the heat of solidification at the contact surface, crystallization is initiated. During that fractionated crystallization, the bed of crystals which is formed in the lower part of the container is subjected to a pressure which is applied intermittently by a vertically moving rammer member. At the end of the crystallization procedure, that is to say, when about 70% of the mass has crystallized, the tapping orifice is opened and about 12% by weight of the initial mass is discharged in the form of mother liquor. A heat flux is applied to the surface of the mass of crystals so as to cause remelting, and then in succession there is removed 16.6% of liquid, with the tapping orifice being fully open, followed by 40%, with the speed of removal being reduced, and finally, the remaining 31.4%. This last removal of material provides a metal containing 30 ppm of silicon and 10 ppm of iron, while the starting material contained 420 and 280 ppm respectively; that corresponds to purification coefficients of 14 for silicon and 28 for iron. As it is known that the coefficients of partition of silicon and iron are about 0.14 and 0.03, it is deduced therefrom that the degree of purification in respect of iron is slightly lower than that corresponding to the partition coefficient (1/0.03=33), but in contrast, purification in respect of silicon is about twice that value (1/0.14.perspectiveto.7).
French Pat. No. 1,594,154 filed in 1968 discloses a purification process which comprises:
- causing progressive solidification within a volume of liquid metal which is maintained in the region of its melting point in an externally heated container, by immersing an internally cooled body therein, PA1 - collecting all the small crystals which are formed, at the bottom of the container, PA1 - causing the formation of large crystals which are about 1 cm in diameter, within which are observed cells, the dimensions of which, being close to 1 mm, led to the assumption that they are the traces of small crystals, during which phenomenon the mother liquor is progressively displaced upwardly in the container, and PA1 - separating the purified large-crystal fraction from the fraction which is enriched in respect of impurities.
As shown in the drawings of this patent, the ratio of the height to the cross-section of the container used is substantially higher than that disclosed in the above-quoted U.S. patent.
Operating examples taken from the French patent may be summarized as follows:
(1) Starting with aluminum containing 320 ppm of silicon and 270 ppm of iron, there is produced a purified fraction representing 70% of the initial mass of aluminum. This fraction contains 20 ppm of silicon (that is to say, a purification coefficient of 16), and 15 ppm of iron (that is to say, a purification coefficient of 18). It may be noted that this operation has a very high yield (70%) resulting in purification coefficients which are themselves very high, that of silicon being higher than that given by the partition coefficient of that element.
(2) Starting with aluminum containing 620 ppm of silicon and 550 ppm of iron, a purified fraction containing 40 ppm of silicon and 10 ppm of iron was produced, the fraction comprising 50% of the initial mass of metal. The stated proportions of silicon and iron in the purified fraction represent purification coefficients of 15.5 in respect of silicon and 55 in respect of iron. It will be seen, therefore, that, in comparison with U.S. Pat. No. 3,303,019, French Pat. No. 1,594,154 makes it possible, with higher yields (50% instead of 30%), to produce a metal with higher purification coefficients: 15.5 instead of 14 in respect of silicon and 55 instead of 28 in respect of iron.
It will also be seen that, in regard to silicon and iron, the purification coefficients are markedly higher than those which are deduced from the partition coefficients. That result is all the more surprising since, as the small crystals which are formed are purer in respect of eutectic elements than the liquid, the liquid becomes enriched in respect of impurities as the crystallization process progresses, which should result in less advanced purification of the crystallized mass.
That result was studied and described in Revue de l'Aluminum (May 1974, page 290) as resulting from a procedure of "successive in situ re-melting steps".
Moreover, a prior publication by one of the authors of French Pat. No. 1,594,154, in Compte-Rendus de l'Academie des Sciences de France (volume 272, page 369, series C, 1971) showed that an impure metal placed in a temperature gradient covering the gap between the solidus and the liquidus tends in a few minutes to assume the state of equilibrium, by a process of melting and solidification phenomena, the state of equilibrium being achieved when, in the gap between the solidus and the liquidus, the metal is completely solid with levels of impurity concentration equal to those given by the solidus at the local temperature.
That shows that the small crystals which are initially formed from the mother liquor have a tendency, after settling, to assume the composition which is given by the solidus at the temperature at which they are. These crystals therefore undergo purification with respect to the initial level of concentration, if they are at a temperature that is higher than the temperature at which they were formed. This is possible since the container in which they are is heated in such a way that, before the cooled body is introduced, the mass of aluminum is in a completely molten state.
U.S. Pat. No. 4,221,590 filed in 1978 uses the same means as those described in U.S. Pat. No. 3,303,019, but with the addition of the step of heating the bottom and the walls of the container. According to this later patent, that partial re-melting step makes it possible to restore equilibrium in respect of the concentration of the small crystals, to improve the levels of performance, and to achieve purification coefficients which are higher than those that are deduced from the partition coefficients. As indicated above, the result was already achieved in the other patents quoted.
However, if we look at FIG. 2 of U.S. Pat. No. 4,221,590, which gives the results of purifying silicon in dependence on the amount of aluminum removed from the crystallization unit, it is found that the improvement over the prior U.S. patent relates in particular to the first 40% of the purified mass, the proportion of which seems to go from 250 ppm to 100 ppm approximately. In contrast, in regard to the remaining 30%, the proportion is substantially the same, in the region of 20 to 30 ppm. It is also stated that very high levels of purity of the order of 3 ppm of Fe are attained, but without specifying the amount of metal on which that result was achieved.
In summary, the two U.S. patents achieve yields and levels of purification inferior to those in the French patent. To the extent that very high levels of purity, higher than those described in the French patent, are achieved, these are accomplished using amounts of metal which are not specified.
To the extent that the complementary means of the three patents make it possible to achieve purification coefficients which are higher than those deduced from the partition coefficients, the part played by the complementary means must be considered. In the French patent, the purified metal is in the form of a compact mass of large crystals containing no, or virtually no, liquid. In contrast, it is stated in U.S. Pat. No. 4,221,590 on the one hand that the deposit of crystals is facilitated by the action of the rammer member which breaks up the massive formations of crystals and, on the other hand, that heating the bottom of the container prevents the liquid phase from congealing on the lower portion thereof. These are details which show that that process involves avoiding the formation of a solid, compact mass and in contrast maintains the presence of an intimate mixture of liquid and crystal.
Therefore, in the two processes concerned, operation is effected using purified masses which are very different in constitution; in the French patent, the mass used is virtually solid and compact, whereas in the U.S. patent the mass used is an intimate and non-compact mixture of solid and liquid.